Communication system



Sept 22, 1936- I R. w. cHEsNUT ET AL 2,054,789

COMMUNICATION SYSTEM Filed May 2e, 1954 2 sheetssheet 1- Nm. mh Wm. WM .om

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COMMUNICATION SYSTEM Filed May 26, 1954 2 Sheets-Sheet 2 Patented Sept. 22, 1936 oNrriazD-lI vsrrirlszs PATENT OFFICE COMMUNICATION SYSTEM lRoy W. Chesnutand Ralph E. Crane, Upper Montclair, N., '.I., and George H. Huber, Brooklyn, N. 'Y., assignors to Bell Telephone Laboratories, j lncor'porated, New York, N. Y., a ,corporationf'of-New York Application May 26, 1934, Serial No. 727,628

` Y 3 Claims.

This invention relates -to multi-channel com munication systems in Vwhich Vthe various channels transmit signal currents at different frequencies over a single communication line and more particularly the case of la communication line which is loaded, or in which the attenuation increases rapidly with frequency or both. Such a system is a carrier current communication system operating over a long submarine cable. The invention, however, is applicable to all multi-channel carrier current communications systems regardless of whether the line furnishes a number of channels all operated in the same direction, four-wire, or twowire two-Way facilities, either `exclusively or in combination. 1

'I'he object of this invention is todecrea'se the interfering energy in relation to the received signal energy. A system operated in accordance with the method herein disclosed as compared to other methods of operatingv transmission systems of the type in question makes it possibleto obtain more or higher grade'l channelslv from a given line or to obtain the Vsame number and `grade of channels from a line of higher attenuation and therefore of less cost. These results may also be considered therefore objects of the invention.

U. S. Patent to H. W. Dudley, 1,832,366, November 17, 1931, discloses a single channel .communication system in which the transmitting volumes of the different frequencies'within `the single frequency band transmitted are adjusted -to obtain lan optimum ratio between the volume of the received frequency and thevolume of the received static 4and other noises having that frequency. 'I'he present invention, however, .-relates to the discovery .that the maximum allowable attenuation in any channel of a multi-channel carrier current .communication lsystem could transmitting volumes to the transmitting curcuit or channel.

.signal in that channel.

rents 4in the diierent channels in accordance with this invention.

,In any signal transmission system, theflowest volume to which signal energy may be allowed to .fall Ais limited by the-volume .of the extraneous i,

noises or energy in the system. The lmargin between the volume of Vthe signal venergy and the noise energydetermines `the grade of the system er .channel-the greater this margin in any circuit or ychan-nel, the higher the grade of the cirtion facilities are designed to furnish vcommuni- .ca-tion 4channels in which the margin .between the signal and interfering energy will meet va termines the minimum allowable volumeof the When a system Vcons-ists of morefthan .one section with repeaters `rconnecting them,the margin between 'signal land noise volumesin the individual sect-ionsmust be greater than for a single section Ysystem furnishing an equal grade of facilities., since the noise i-n each section .is repeated and therefore adds to the..

,.10 In general, new communica-V total-amount. The lkdiscussion which fo1lowsap" `yplies to :asingle section of a system :containing one or more sections, .the assumption being made `that rsatisfactorymargins will -be provided in a multi-sectionfsystem, so that .each Vsection may."

be Vconsidered by itself as a Vsingle section system.

The various types of interference -may be divided -in-.two classes.: (l) Thosenot dueto operation, such `as static, thermal agitation (called re- .qsistance-noise), :crosstalk from other signal lines` carrying similar `frequency bands and induction from power lines; :and Y(2) unwanted frequenices not-containedin Athe original signals but which areiset up by signal energy applied `to any .chan- Thelatter are,

nel -orfccmbination of channels. ,caused 'by fthe non-.linear characteristics vof, the

lines or terminal equipment which result in the Yproduction of harmonics -of individual frequen- -cies as `well :as :of `frequencies which ,arey func- -tions :of two or more frequencies simultaneouslyk applied :to :the l-ine. The l.process of interaction just described is ,known :as modulation and the lresulting unwanted vfrequencies are modulation vproducts. f

The maximum volume at which a signal may be Aapplied to -a channel ina multi-channel cominunication system is limited not so ,much by a f distortion and thefgenerationof rmodulation products .in theiparticular channel concerned, as by V,the-volumes 01 the modulation products fallingspecified value, depending upon :the grade of 1.,5 `circuit desired. margin, in conjunction withV the volumeynf the noise in a given channel, de-

ence in the other channels in which Very low.

Volume energy is being received from the same terminal of the line. J

In the simple case of a two-channel system it is necessary to consider .only nthe 'modulation products in one channel resulting from energy ow in the other. In considering the case of more than two channels, the matter is simplified by the e fact that the combined interfering eiect ofn two or more channels operating intheA same direction of transmission is approximately equivalent inv 4 effect to the eiect of one such channel carrying energy equal to the total energy Yinthose channels.

In the case of a multi-channel communication system, the most efcient use of the line requires that the receiving volume of the band having the greatest attenuation be limited only by interference of the iirst type previously discussed. This requires that the modulation Vproducts in this channel, due to energy applied to other channels, shall not increase the noise level materially. Due to the intermittent nature of the modulation products as contrasted to the relatively steady nature of noise due to thermal agitation (resistance noise) the volume of Vmodulation products in any speech band often may be permitted to be nearly as large as that of the resistance noise. For many cases, a volume four decibels lower than that of the resistance noise is satisfactory. Y l e With the permissible Volume of modulation products in the channel of highest attenuation as a starting point and with information regarding the relation between the volume of the modulation products produced in a given channel and the energy volumes applied to other Ychannels and knowing the ratios between the 'attenuations of the various channels, the maximum transmitting Volumes of the channels may be determined as described more in detail hereinafter. The difference between the sending 'and receiving volumes, expressed in decibels, of the channel having the highest attenuation will then be equal to the greatest attenuation that can be allowed for the desired number and grade of facilities. It follows therefore that a line built to have this maximum attenuation may be expected to cost less than any other that will give equally satisfactory performance. Y

As to the relation between the' volumes of the modulation products and those of the applied energy causing the modulation products, the results of a number of investigations show that a change in the total volume of the applied energy, expressed in decibels, above or below a suitable reference volume causes a greater change in the volume, also expressed in decibels above or below the reference volume, of lthe modulation products. The ratio between such changes may be designated'by the symbol a, a being, in general, greater than unity. It is found that a is substantially constant (for a given line) over the ranges of applied energy volumes usually encountered. From the foregoing, it will be seen that the volume M of the modulation products in a particular channel willbear a denite relation to the volume I of the applied energy and in another channel when -the volumes are expressed in terms of decibels above or below a suitable reference volume. YIf the applied energy is adjusted to reference volume, the volume of the modulation products will be a definite number of decibels lower, depending upon the; type of line or equipment causing the modulation and the frequency bands of the channels' concerned. This difference in Volume is called the modulation constant, MC, for the particular conditions concerned. For various values of I,y the volume of the modulation products lVf will be equal to aI-Mc. For someV other channels affected by modulation products due to energy -applied to another group of channels, the ratio and modulation constants may be different, although not radically so, and we may have The ratios 1c, k1, k2, etc., between the attenuations of thevarious channels may be approx- `imated by using the ratios obtained from the Athe size of cable indicated by the result of the application of this procedure.

As an example of the determination of the optimum operating volumes, as well as of the maximum attenuation that will permit a specified grade of transmission, the case of a line carrying only two channels will be described first. For simplicity, it may be assumed that both channels are operated on a two-way basis, thus furnishing two independent circuits and requiring that the interference from one channel into the other be Very small.

The specied margin between speech energy and noise may be designated as S, expressed in decibels. Assuming that the cable or line and terminal apparatus is adequately shielded from extraneous interference, the noise volume in the channel havingthe highest attenuation may be determined from fundamental data on resistance noise and ampliiier noise. This volume may be designated as N decibels below reference volume, i. e., -N. With the margin of four decibels between noise and modulation products, as described above, the latter must not be higher than -N-l. If this volume is due to the application of energy at the volume I in the other channel, then it follows that The above expression gives the input volume of the channel of lesser attenuation, which is the lower of the two frequency bands under consideration, since the attenuation increases with frequency. Y If the attenuation of the higher channel is designated by A, expressed in decibels, the attenuation of the lower band will be kA. The receiving volume in the lower band will .be I ICA and the allowable level interference will be I kA-S. Where S is the specified margin between the volurne of the received signals and the noise and modulation products, if the input level of the upper channel is designated by I', it follows that:

VVI

`put volumes is much the same.

'In this expression Vonly'I` and A are unknown. However, I may be expressed in terms of A, since the input volume ofthe upper channel is A decibels above the receiving volume, which iss decibels above the noise volume which is --N,V soV that:

Y Y I :Al-S -N (4) Now, susbtituting the values for I and I from (2) and (4) in (3) and solving for A, we have:

lThis is the highest attenuation of any channel (usually the channel operating in the highest frequency range) of the system and by substituting it in (2)` and (4) the transmitting volumes may be easily obtained.

In a multi-channel carrier system having more than two channels the procedure for determining the maximum allowable attenuation and the in- Modulation products from channels which operate always in the same direction as the particular channel under consideration usually may be neglected, as may also the products from the opposite bound channel which forms the other half of the same twoway circuit. It is necessary, however, to consider the effect of all other opposite bound channels transmitting simultaneously while speech energy is being received onV the channel in question. This may be done by assigning `differences in volume roughly proportioned to the attenuations of the channels and considering the combined volume as being concentrated in one channel. With a little experience it is practicable toselect the two channels which have the greatest ini'iuence in determining the maximum allowable attenuation. After determining the attenuation by the procedure herein described, the effects of thevarious channels upon each other may be checked and such minor adjustments made as will permit the attenuation to be increased to the maximum value. The chief adjustment applies to the assignments of the differences in volumes of the like-bound channels.

- By following the procedure described above and repeatedly adjusting the assumed values to be consistent with the result of the determination,

`the final determination will give attenuation and volume data from which the final design rof calble structure may be worked out and the optimum operating conditions secured, with the result that the cable will cost the least of any that will give the desiredv number of circuits, each of which will be of substantially thesaine grade.

In order tofurtl'ier illustrate the principles of this invention, its application to a typical multichannel carrier communication system Will now be described with reference to Figs. 1 and 2. L1, L2 and L3 represent three communication channels which transmit and receive signals to and from lines L1, Lz and La, respectively. Since all the lines are substantially equivalent, the detailed structure will be described for lines Li and L1 only. Starting with L1 the signals enter the hybrid coil I and' pass through to modulator II.

` pass filters.

The balancing network45 provides an impedance balance with the line L1 so that signals from demodulator 44 will not be transmitted to modulator II. The incoming signals from line L1 therefore pass to modulator I I and are modulated thereby and are then amplified by ampllner I2. YThey then pass through the band pass filter I3 to the equalizer I4. Signals from the other lines Le and L3 pass through similar equipment to equalizer I4 Where they all pass through equalizer I4, amplier I5 and directional iilter I5 to't'ransformer I1 and then over line I8 to terminal transformer I9, directional filter 36, amplier 35, equalizer 34, supplementary lter 33, amplifier 32 to band pass iilterswhere they are separated -to their respective channels.V The signals from channel L1 are selected by band pass filter 23 and then pass through amplifier Zfl, demodulator .25 andrhybrid coil 2B to= line L1.

In a similar manner, signals from line L1 pass through hybrid coil 25, modulator 28, amplifier 30, band pass lter 3l where they are joined by similar signals `from lines Lz and La. All the signals then pass through equalizer 22, amplifier 2 I, directional lter 25, line transformer I 9, lineIS, terminal transformer I1, directional filter 3l, amplifier 38, equalizer 39, supplementary filter itl and amplifier 4I Here the signals from L1, Lz and La are again directed to their respective lines by band 'I'hus the signals from L1 pass through filter 42, amplifier 43, d'emodulator 54 and hybrid coil I0 to linefLi. Fig. 1A shows specific details of line I8 when it is a submarine cable. In this case, transformer I'I, line I8 and transformer I9 are replaced by transformer 43, condensers'l, submarine cable 90, sea earth cable 5I) and balancing network 52, condensers 48, transformer 49, sea, earth cable 5l and balancing resistance 53. If desired, loading material 55 may be applied to either the submarine cable 33, sea earth cables 59 and 5I or to both submarine cable and balancing sea earth cable as is desirable. Although the invention is shown as applied to a sea earth connection, it may be equally well applied to a concentric return cable system.

Thus, in this system shown in Figs. l. and 1A `there are six channels employed, 'three of which transmit currents in one direction while the other three transmit currents in the opposite direction and are combined to form three two-way, fourvlire channels.

In designing and operating such a system in accordance with this invention, the noise conditions and variations of attenuation of the cable with frequency are rst'investigated. The attenuation versus frequency characteristic` of such a cable is illustrated by c urve 50 of Fig. 2. The attenuation of the cable is, of course, positive. Thus curve- G0 shows the output Volume from the cable when a reference volume input is applied to the input of the cable. A typical curve showing the level of the natural noises by resistance and interference versus frequencyof such a system` is shown by curve 6I of Fig. 2. Next, the number and width of the frequencybands are selected and assigned to the appropriate channels. Thus, in Fig.

2 the three lower channels transmit the signals East, that is, from lines L1, L2 and L3 to lines L1, L'2 and La, respectively, While the three upper frequency bands carry the signals in the reverse direction from L1, Lz and Ls to Li, L2 and L3, respectively. Then some reference frequency, usually the frequency which corresponds to the 1000 cycle frequency of the original current is chosen in each channel. These frequencies are designated by lines 62, 63, 64, 65, 66 and 61 in Fig. 2. Then, from Equation (6) maximum attenuation A for the highest channel, that is, channel No. 3, may be determined since N is given by curve 6 I Mc and 7c are constants of the cable structure and S is spe/cied or determined by the required performance ofthe system. Then, having determined ths factor, A the transmitting volume in the lower channels is next determined. Since, as pointed out above, the modulation products in the highest channel should not exceed the noise level but should be at least four decibels below this noise level, as shown by the circle at B8 in Fig. 2, the sending level in the lower two channels, channels No. l-East and No. Z--East may be determined by Equation (2). It must, however, be remembered that this volume is the combined volume of both of these channels transmitting at the same time. In a similar manner the transmitting volume for the topi channel, channel No. B-West may be determined in accordance with Equation (4) In a similar manner the transmitting levels in the other channels may easily be determined. These volumes are shown by lines 14, 15, 16, 11, 18, and 19 in Fig. 2. The receiving levels then may be determined by subtracting the attenuation, of the cable from these transmitting volumes and are shown by lines 80, 8|, 82, 83, 84, and 85. Then, by subtracting the specied margin S from these receiving volumes the permissible noise level is obtained and is illustrated by the short horizontal lines at 13, 12, 1|, 10, 69, and 68 of Fig. 2. It is notedV that this permissible noise volume is above the natural noise shown by curve El and the modulation products which are shown by the circles also designated by 13, 12, 1I, 10, 69, and 68 and also the combined volume of this noise and the modulation products which are illustrated by the cross at 13,12, 1I, 10, 69, and 68 of Fig. 2. As shown at 13, there is a considerable margin between the cross and the horizontal line showing that the actual noise volume is somewhat below the permissible volume of this noise so that this channel provides a higher grade circuit. If it is desired, or if this margin should be too large in any channel, the transmitting volumes may be again readjusted in accordance with these formulae by slightly redesigning the cable and changing the constants Mc and 7c as pointed out above, until all the channels provide substantially the same grade of transmission.

If circuits of different grades are desired, suitable values of S may be used for the particular channels concerned and data obtained permitting the design of the most economical cable for the The above description which describes several specific embodiments of the invention merely illustrates the features of the invention but does not restrict or limit the scope of the following claims which define the invention.

What is claimed is:

1. A loaded submarine cable of greatly varying attenuation with frequency designed to transmit a plurality of bands of frequencies, each band correspondingV to a different channel of communication, in combination with terminal apparatus for applying message currents of those frequencies thereto, characterized in this, that said terminal apparatus includes means for applying the message currents to the respective bands at such a Yvolume that the Yreceiving volume in each band is at a substantially uniform margin above the combined noise induced by currents in other bands and noise produced from all other sources.

2. In a multi-channel carrier current deep sea communication system in which message currents of a plurality of bands of frequencies, each band providing a communication channel, may be'simultaneously transmitted in both directions o-ver said various channels comprising a submarine cable of varying attenuation with frequency, loading means incorporated in said cable for reducing the attenuation'of said cable, terminal apparatus for transmitting and receiving message currents to and from said cable, means for transmitting the message currents to the various channels at such volumes that the modulation products induced into the higher attenuation channels when combined with the resistance noise will not appreciably increase the volume of the `interference over that due to resistance noise alone, and such that a specified margin is maintained betweenV the message current volume and the volume of the combined noise and modulation products in each channel.

3. In a multi-channel carrier current deep sea communication system in which message currents of a plurality of bands of frequencies, each frequency band of which provides a communication channel, may be simultaneously transmitted in the same and opposite directions over the various channels comprising a submarine cable, the attenuation of which increases rapidly with frequency, terminal apparatus for transmitting and receiving message currents to and from said cable, means for increasing the maximum allowable attenuation which comprises means for adjusting the transmitting volumes of the various channels so that modulation products induced into the channels having the higher attenuations when combined with the noise of these channels will not appreciably increase the volume of the interference in the channels, and so that a specified margin is maintained for each channel between combined volume of the noise and the modulation products and the volume of the message currents therein.

ROY W. CHESNUT. RALPH E. CRANE. GEORGE H. HUBER. 

